1. Field of the Invention
The present invention relates to a tool position offsetting system and more particularly to a tool position offsetting system for a spare tool which is used for the replacement of a broken or substantially worn tool.
2. Description of the Prior Art
In a conventional machine tool a measuring device is provided for detecting a broken or substantially worn tool and is capable of initiating an action which results in the replacement of the broken or substantially worn tool by directly replacing the same by a spare tool before the next operation is effected. Such a device is disclosed in U.S. Pat. Nos. 3,817,647 and 4,185,376. Further, in another conventional device such as disclosed in U.S. Pat. No. 3,626,266, offset storage means are provided for offsetting the tool position of a tool.
However, in such conventional devices, it is impossible to offset automatically the tool tip position of the spare tool, since the offset storage device is not available for a spare tool. Therefore, it has been required to preset exactly the spare tool at the beginning of the machining operation by the time consuming process of manual setting. Otherwise, the machining cycle must be suspended in order to input manually the tool offset data for a spare tool in the offset storage device when a broken or worn tool is replaced by the spare tool.
More particularly, in a conventional numerical controller, there is a memory having a plurality of memory areas, each of which stores tool offset data for a tool. When an NC program is fed into the numerical controller by a tape reader, one of the memory areas is designated by a D code in the NC program so that a desired tool offset data is read out therefrom for the tool position offsetting operation.
However, when a broken or worn tool is replaced by a spare tool, the D code designating the memory area is not automatically changed. Therefore, the tool offset data for the spare tool is not read out from the corresponding memory area and the tool position offsetting operation for the spare tool is not carried out. Accordingly, in the case that the position of a cutting tool tip of the spare tool does not precisely coincide with the position previously set in the NC program, the machining cycle must be stopped in order to input manually the tool offset data for the spare tool in the offset storage device.